Heat integrated distillation column

ABSTRACT

The invention is directed to a heat integrated distillation column comprising a cylindrical shell having an upper and a lower end and at least one first inner volume and at least one second inner volume in the shell, and being in heat exchanging contact with each other through a wall separating the volumes, the improvement comprising providing means having heat exhanging capacity extending through the wall from the at least one first volume into the at least one second volume, whereby the inside of the the heat exchanging means is in open connection with the said first volume.

The invention relates to a heat integrated distillation column havingseparate volumes inside the column, which is especially suitable fordistillation operations in the process industry. More in particular theinvention relates to such a column, wherein the said volumes can beoperated at different temperatures with improved heat exchange, therebyproviding energy advantages in the operation.

It is well recognised that heat integration in distillation columns isan important means for providing improvements in energy efficiency inthe operation of distillation. However, the application of thistechnology has been impeded by factors of cost of construction and thedifficulty of providing adequate heat exchange, especially withoutcomplicated construction of the column(s).

In U.S. Pat. No. 4,681,661 a heat integrated distillation column hasbeen described, which column comprises a central column, and an outer,annular column around the central column. Thereby different regions areprovided in the column, which regions can be operated at differentpressures. Both regions are provided with conventional trays anddowncomers.

In U.S. Pat. No. 5,783,047 a heat integrated distillation column hasbeen described, which column comprises an outer shell and inside one ormore tubes Thereby different regions are provided in the column, whichregions can be operated at different pressures. However, in order toprovide sufficient heat exchange area between the two regions inindustrial large scale operations, several tubes of relatively smalldiameter have to be placed in the outer shell. Due to the relativelysmall diameter of the tubes, the use of distillation internals insidethe tubes is limited to irregular packing rings or structured packing.The use of trays requires a complicated construction.

In U.S. Pat. No. 4,234,391 a continuous distillation apparatus andmethod has been described, wherein a column has been divided into twoseparate semi cylindrical sections by a dividing wall, one sectionfunctioning as stripping section and one as rectification section. It isan object of the present invention to provide a heat integrateddistillation column, consisting of two separated volumes along thelength of the column, wherein sufficient heat transfer is providedbetween the volumes. It is also an object of the invention to providefor a heat integrated distillation column of this type, wherein trayscan be used.

This object and other objects are provided for by the column of theinvention. This column is a heat integrated distillation columncomprising a cylindrical shell having an upper and a lower end and atleast one first inner volume and at least one second inner volume in theshell, being in heat exchanging contact with each other through a wallseparating the volumes, the improvement comprising providing meanshaving heat exchanging capacity extending through the said wall fromsaid at least one first volume into said at least one second volume,whereby the inside of the said heat exchanging means is in openconnection with the said first volume. Of course the heat exchange meanshave no connection for mass transfer to the other (second) volume.

The important aspect of the column of the present invention resides inthe presence of means for providing heat exchange, which means extendinto the other volume, thereby providing for the possibility of heattransfer from the one volume to the other volume, resulting in partialcondensation of vapour in the hotter (usually high pressure) section and(partial) evaporation of liquid in the cooler (usually low pressure)section.

The heat integrated distillation column of the invention preferably hasan enriching section and a stripping section, one of the volumes beingthe enriching section and the other being the stripping section. Whenthe terms ‘enriching section’ and ‘stripping section’ are used hereinthey are also to be considered referring to the separate volumes of thecolumn.

The heat integrated distillation column of the invention has aconstruction in which the enriching (rectification) section (E) (portionabove the feed stage) and the stripping section (S) (portion below thefeed stage), as encountered in a conventional distillation column areseparated from each other and disposed in parallel, and the operatingpressure of the enriching section is made higher than that of thestripping section so that the operating temperature of the enrichingsection becomes higher than that of the stripping section. In thisconfiguration, if there exists a heat transfer surface between them,heat transfer occurs from the enriching section to the strippingsection. In the heat integrated distillation column of the invention theheat transfer occurs from the enriching section to the strippingsection.

The invention can be seen in two preferred embodiments. In the firstembodiment the heat exchange means are located in the cooler section andvapour is introduced into the heat exchange means from the hottersection and condenses in the heat exchange means, thereby giving offheat to the cooler section. The condensed vapour (liquid) is returned tothe hotter section. On the outside of said heat exchange means, liquidis evaporated.

In the second embodiment the heat exchange means are located in thehotter section and liquid from the cooler section is passed into theheat exchange means. Said liquid is (partially) evaporated inside theheat exchange means and vapour (partially) condenses on the outside ofthe said heat exchange means. The vapour generated in the heat exchangemeans is returned to the cooler section. In general it is preferred tohave liquid film flow in both embodiments.

In the heat integrated distillation column of the invention, in both ofthe volumes, vapour which enters from the lower end and goes out of theupper end comes in contact with liquid which enters from the upper endand flows to the lower end, on the surface of the packing or on thetrays. At this time, the mass transfer occurs, and hence thedistillation operation is performed. In the heat integrated distillationcolumn of the invention, two distillation sections, i.e., ahigher-pressure section and a lower-pressure section are disposed in onecolumn.

In contrast to the conventional distillation column in which the heatinput is provided by a reboiler, according to the heat integrateddistillation column of the invention, the heat input is mainly providedin the whole of the stripping section, with the result that the heatload on the reboiler can be minimised. In the conventional distillationcolumn, the heat removal is performed by a condenser disposed at the topof the column. In contrast, according to the heat integrateddistillation column of the invention, the heat removal is performed inthe whole of the enriching section with the result that the condenserduty can be minimised. Accordingly, it is possible to save aconsiderable amount of energy, compared with conventional distillationcolumns.

In a heat integrated distillation column, vapour is condensed in theenriching section, and hence the flow rate of the vapour is decreasedtoward the uipper portion and liquid is vaporised in the strippingsection, so that the flow rate of the vapour is increased toward theupper portion. Therefore, in order to ensure that the ratio of thevolume flowrate of the ascending vapour and the cross-sectional area ofthe specific volume is kept within the operating range of columninternals irrespective of the height of the column, the volumecross-sectional area should be decreased when moving from the bottom tothe top of the enriching section, and increased when moving from thebottom to the top of the stripping section. This aspect of a preferredembodiment of the invention has been shown in the figures, which will bediscussed below.

The column of the invention may be constructed in various ways, providedthe two volumes are always adjacent to each other, divided by aseparating wall. In practice this means that two possibilities arepreferred. The first possibility is a column, having a concentric innercolumn. The other possibility is a column provided with a dividing wallthat reaches from one side of the column to the other side.

The column of invention contains means for improving vapour/liquidcontact, which means can for example be trays, which is preferred, butalso random or structured packings. It is not necessary to have the samesystem of said means for improving vapour/liquid contact in bothvolumes.

As indicated above, preference is given to the use of trays withdowncomers, as these provide an easy and uncomplicated way of providingvapour/liquid contact. In this embodiment the means for heat exchange,preferably vertical heat transfer panels, are provided in the downcomer,and the liquid that flows down is distributed over the surface of thepanels by means of liquid distribution systems.

The means having heat exchanging capacity can have the form of plates ora tubular construction. The surface of the plates or tubes can be smoothor textured. In general it is possible to use coils, flat plates, dimpleplates, finned plates or finned tubes, corrugated plates or other platesthat enhance heat transfer.

In general it is preferred that there are vapour-liquid disengagementmeans present in, in between, around or above the heat exchange means,to improve separation of vapour from liquid. Suitable means are fins,vanes, corrugated structured packing sheet, dumped packing and thelike.

The heat exchange means extend through the wall from the first volumeinto second volume, whereby the inside of the said heat exchanging meansis in open connection with the said first volume.

In a first embodiment of the invention, the heat exchange means are inopen connection with the section having the highest temperature (theenriching section) and vapour enters the heat exchange means from theenriching section and condenses inside. The heat is transferred throughthe walls into the second volume (the stripping section), where liquidevaporates on the outside surface of the heat exchange means. Thecondensate flows back into the enriching section.

In the second embodiment, the heat exchange means are in open connectionwith the section having the lowest temperature (usually the strippingsection) and liquid enters the heat exchange means from said volume andis partially vapourised on the inner surface of the heat exchange meansby heat transferred through the wall of the heat exchange means from thesection having the highest temperature (the enriching section). In thissection vapour condenses on the outside surface of the heat exchangemeans. The remaining liquid flows back into the stripping section, aswell as the vapour.

The present invention is especially suitable for use in energy intensivedistillation operations. Examples thereof are liquid air distillationand the various separations in the petrochemical industry, such asethane/ethylene separation, propane/propylene separation,butane/isobutane separation, air separation, distillation to breakazeotropes and the like.

An important aspect in the invention is the difference in operatingpressure between the two volumes. In order to obtain such differencemeans have to be present to increase the pressure of the vapour streamgoing from one volume to the other volume (such as a blower or acompressor). The pressure in the enriching (or rectification) sectionwill be higher than the pressure in the stripping section. In generalthe ratio of the pressures will not be much higher than that requiredtheoretically to obtain sufficient amount of vaporisation of the liquidin the stripping section. In general this ratio will not exceed 2.

The invention will now be elucidated on the basis of a number offigures, wherein preferred embodiments of the invention will bedescribed. These figures are not intended as limiting the scope of theinvention.

DESCRIPTION OF FIGURES

FIG. 1 shows a top view of a tray in a concentric column according to anembodiment of the invention, which column has been fitted with trays anddowncomers,

FIG. 2 shows a vertical cross-section of the column depicted in FIG. 1,along the line A-A,

FIG. 3 shows a top view of a tray below that depicted in FIG. 1,

FIG. 4 shows a vertical cross-section along the line B-B in FIG. 3,

FIG. 5 shows a possible configuration of the liquid distribution systemin a three-dimensional drawing,

FIGS. 6 a-b-c-d show a possible assembly of heat transfer panels,

FIG. 7 shows a top view of a column of the invention based on a flatwall dividing the column into two volumes,

FIG. 8 shows a cross section of the divided wall column,

FIGS. 9 and 10 show the feature that the ratio of the cross sectionalareas of stripping and enriching sections varies with the volume ofvapour along the height of the column.

FIG. 11 shows a vertical cross-section of a column according to afurther embodiment of the invention,

FIG. 12 shows various cross-sections of heat exchange means suitable foruse in the embodiment of FIG. 11, and

FIG. 13 shows an enlarged cross-section of heat exchange means of FIG.12.

DETAILED DESCRIPTION OF FIGURES

FIG. 1 shows a top view of a tray and FIG. 2 a vertical cross section ofa part of the column according to an embodiment of the invention,wherein the heat exchange means are in open connection with the volumeof the highest temperature. The cross section shows 4 trays (a, b, c, d)in the inner column and 4 trays in the annular outer column. The topview refers to tray (a) as indicated in the cross section. The trays canbe either sieve trays or any other type used in industrial distillationsuch as valve trays, bubble cap trays, or tunnel trays. The dashed lineson the top view drawing show the downcomers positioned above the tray.

Tray (a) of the inner column is of ordinary cross flow design andprovided with rectangular downcomer pipes. The arrows indicate thedirection of the liquid flowing over the tray. The liquid exiting thedowncomers from the tray above enters tray (a) on the right-hand side,flows over the tray and is then collected in the downcomers on theleft-hand side.

In this example the trays in the outer annular column are provided withfour downcomers in which the heat transfer panels are mounted. Theliquid exiting a downcomer from the tray located above tray (a) splitsup at the outlet into two equal portions each entering the active areaof tray (a). The arrows indicate the direction of the liquid flow overthe tray. At the end of the active area section the liquid is collectedin main troughs, which are positioned above the downcomers. Thesetroughs are provided with side channels that enable the distribution ofthe liquid over the heat transfer panels.

The cross section drawing at location A-A shows the position of thetrays and the heat transfer panels. The top of the heat transfer panelsis connected via one or more tubes to the vapour space of the innercolumn. At the bottom the heat transfer panels are provided with a tubefor drainage of the condensate to the tray of the inner column.

FIG. 3 shows a top view of tray (b) that is located direct below tray(a) and FIG. 4 the cross section B-B. The position of the downcomers inthe outer column has been rotated over an angle of 45° with respect tothe tray above. In case the trays of the annular column are providedwith 2 downcomers this rotation angle will be 90° and in case of 6downcomers 30°.

FIG. 5 shows a three-dimensional drawing of a possible configuration ofthe liquid distribution system placed above the heat transfer panels inthe downcomers of the stripping section. The liquid flows via the maintroughs into the side channels. In the walls of the side channels holesare provided to distribute the liquid over the heat transfer panels. Atthe outside of channel walls the holes are covered with splash plates toensure a film flow of liquid over the heat transfer panels. For thisreason the splash plates extend over the top of the heat transferpanels. At the end of the channels weirs are provided to maintain aconstant liquid level. Excessive liquid is discharged over these weirs.

In FIGS. 6 a-b-c-d a possible assembly of heat transfer panels is shown.In this example the assembly consists of 6 parallel panels. The panelsare preferably constructed of corrugated sheets oriented in verticaldirection. Other constructions like coils, flat plates, dimple plates,finned plates or other plates that enhance heat transfer are possibletoo. The FIG. 6 d shows that by the corrugations vertical channels areobtained. At the top these channels are connected to a vapour inletchannel. The six vapour inlet channels are connected to a header withtwo vapour inlets. In a similar way the condensate is drained into theinner column at the bottom of the panels via liquid outlet channelsconnected to a liquid collection header

FIG. 7 shows a top view and FIG. 8 shows a cross section of a column ofthe invention based on a wall dividing the two volumes. In these figuresthe same features are shown as in the FIGS. 1-4.

FIGS. 9-10 show the feature that ratio of the cross sectional areas ofstripping and enriching sections varies with the amount of vapour alongthe height of the column. This has been shown for two possibleconstructions. In FIG. 9 a single cylindrical shell column separated bya divider into two semi cylindrical volumes is shown. The crosssectional area of both enriching and stripping section is changedstepwise. FIG. 10 shows the stepwise cross sectional area variation fora concentric column.

FIG. 11 shows a vertical cross-section of a column according to a secondembodiment of the invention, wherein the heat exchange means are locatedin the central (enriching) section and are in open connection with theannular (stripping) section. As can be seen in the figure, liquid entersthe tubular means from a tray and flows down, preferably as a film,inside the tube. Part of the liquid evaporates inside the means andrises. The vapour flows from the top of the heat exchange means into theannular section, whereby said means preferably have liquid-gasdisengagement means to provide a proper gas-liquid separation. Theremaining liquid that is not evaporated flows back into the strippingsection from the bottom of the heat exchange means.

FIGS. 12 and 13 shows various cross-sections of en example of a suitableheat exchange means for the embodiment described in relation to FIG. 11.In the figures (a), (b) and (c) indicate the various connections of theheat exchange means with the annular section. (a) is the connectionthrough which the unvaporised liquid flows back into the annularsection, (b) is the liquid entry and (c) is the vapour removalconnection. (d) is a possible form of vapour-liquid disengagement means.

EXAMPLE

A column in accordance with the construction of FIGS. 1-5, having panelsin the downcomers and the constructional details in Table 1 is used fordistillation of the system propane/propylene. The overall heat transfercoefficient is 700 W/m²K and the heat transfer area per tray is 10.5 m².TABLE 1 Diameter outer column 2.5 m Diameter inner column 1.2 m Trayspacing 0.5 m Length heat exchange panels 0.55 m Height panels 0.4 mHeat transfer area per panel 0.44 m² Number of panels per tray 24 Numberof panels per downcomer 6

For the same type of column as above, but using tubes as heat exchangedevice, the corresponding dimensions are as follows. TABLE 2 Lengthtubes (hairpins) 0.5 m Diameter tubes (external) 20 mm Pitch(rectangular) 30 mm Tubes per downcomer 84

1. A heat integrated distillation column comprising a cylindrical outershell having an upper and a lower end and at least one first innervolume and at least one second inner volume in the shell, and being inheat exchanging contact with each other through a wall separating thevolumes, the improvement comprising providing means having heatexchanging capacity extending through the said wall from said at leastone first volume into said at least one second volume, whereby theinside of the said heat exchanging means is in open fluid connectionwith the said first volume.
 2. The heat integrated distillation columnaccording to claim 1, wherein the said column is provided with an innertube which is concentric with the outer shell, thereby defining a volumeinside the inner tube and an annular volume between inner tube and outershell.
 3. The heat integrated distillation column according to claim 1,wherein the said first and said second volume have been created byseparating wall extending along the inside of the outer shell, andconnected at both ends to the outer wall.
 4. The heat integrateddistillation column according to claims 1-3, wherein said first volumeis constructed to act as stripping section and said second volume asenriching section.
 5. The heat integrated distillation column accordingto claim 1, wherein the heat exchange means are present in the volumethat has been designed as the volume with the highest temperature and isin open connection with the volume designed to have the lowesttemperature.
 6. The heat integrated distillation column according toclaim 1, wherein the heat exchange means are present in the volume thathas been designed as the volume with the lowest temperature and is inopen connection with the volume designed to have the highesttemperature.
 7. The heat integrated distillation column according toclaim 1, wherein vapour disengagement means are present.
 8. The heatintegrated distillation column according to claim 1, wherein the bothvolumes are provided with trays and downcomers.
 9. The heat integrateddistillation column according to claim 1, wherein the enriching sectionis provided with trays and downcomers and the stripping section isprovided with structured or random packing.
 10. The heat integrateddistillation column according to claim 1, wherein the stripping sectionis provided with trays and downcomers and the enriching section isprovided with structured or random packing.
 11. The heat integrateddistillation column according to claim 1, wherein both the strippingsection and the enriching section have been provided with a structuredand/or a random packing.
 12. The heat integrated distillation columnaccording to claim 1, wherein the said heat exchange means comprise apanel or a tubular construction.
 13. The heat integrated distillationcolumn according to claim 1, wherein a plurality of said means havingheat exchanging capacity is present along the length of the column. 14.The heat integrated distillation column according to claim 1, whereinthe said means having heat exchanging capacity are located in thedowncomer of a tray.
 15. The heat integrated distillation columnaccording to claim 1, wherein the heat exchange means are locatedbetween the trays.
 16. The heat integrated distillation column accordingto claim 1, wherein the volume of one section increases from the lowerend to the upper end and the volume of the other section simultaneouslydecreases from the lower end to the upper end.
 17. The heat integrateddistillation column according to claim 1, wherein means are present forproviding a pressure difference between the said first volume and thesaid second volume.
 18. A process for distilling liquefied air, anorganic mixture, or an aqueous mixture, said process comprisingdistilling liquified air, an organic mixture or an aqueous mixture usinga column according to claim
 1. 19. The heat integrated distillationcolumn according to claim 7 wherein said disengagement means areselected from the group consisting of fins vanes corrugated structuredpacking sheet and dumped packing rings.
 20. The heat integrateddistillation column according to claim 1, wherein said heat exhangemeans comprises corrugated sheets oriented vertically, a coil, a flat ordimpled plate, a dimpled tube, a finned-tube, or a finned-plate.